Prosecutors said Owenby plotted with Garrett to retaliate against Agent Matthew H. Thompson, who he blamed for his state conviction on drug charges. Owenby was convicted in October 2012 in Cocke County Criminal Court of selling oxycodone.

Court documents show evidence of intent to harm Thompson was gathered from telephone conversations between Owenby and his mother — neither apparently aware jail phone calls are routinely recorded.

Garrett was arrested Wednesday. Owenby was already held in jail on his drug conviction.